test-lib: consolidate naming of test-results paths

test-lib: consolidate naming of test-results paths

There are two places where we strip off any leading path components
and the '.sh' suffix from the test script's pathname, and there are
four places where we construct the name of the 't/test-results'
directory or the name of various test-specific files in there.  The
last patch in this series will add even more.

Factor these out into helper variables to avoid repeating ourselves.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>